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What is this below the automatic air vent valve?

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fullel

The (apparently rotated) picture shows my badly scaled up automatic airvent valve just a few cm downstream from the pump. I have some water hammer (hydraulic shock) when the pump goes off and this scaled up air vent is my prime suspect. I am ready to drain the system down and screw this old vent off. By the look of the amount of nipple, I will have to cut the olive off, clean the pipe and replace with the new compression 15mm auto air vent.

What I don't know is what the object is immediately below the air vent sticking out to the left and whether I need to replace that too. Is it a manual bleed valve for example?

Thanks.
 

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Left over from old system, probably solid fuel BB, what boiler do you have now ? if you have a pressurised system you will have a more up-to-date PRV, so you can cut out and remove this old fitting
 
Ah yes. Well, still don't know why there are two. So, just replacing the top one should work then? I kind of thought the water hammer was caused by air not being able get _in_ to fill the 'vaccuum' when the pump stops.
 
The top one is an Auto Air Vent. It vents air out of the system automatically (see what I did there?) but doesn't. or shouldn't, allow air back in.

The bottom one is a Pressure Relief, or Safety, valve. It automatically releases pressure over a given point which, in your case, is 3 bar.

Neither the failure, nor the operation, of either component would result in the noise you say you have.

I'd suspect a sticky flapper type none return valve to be honest.

They have two completely different yet vital functions
 
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